In the formulation of modern detergent compositions, ganular STP has come into widespread use as a phosphate "builder" which increases the cleaning ability of these detergent compositions. In formulating free-flowing, homogeneous detergent compositions containing a granular STP product, the STP employed has been manufactured so that its various physical properties suit the ultimate use. Physical properties which are generally important include bulk density, frangibility and absorptivity. U.S. Pat. No. 4,255,274 discloses a method of preparing granular STP, wherein dense rotary-dried STP is moisturized by adding from about 10% to 30% by weight water, then calcining at a temperature in the range of about 300.degree. to 600.degree. C. U.S. Pat. No. 4,251,498 discloses a method of preparing granular STP products wherein screen undersize STP particles are compacted, milled and moisturized by adding from about 15% to 35% by weight water, and then calcined at from about 300.degree. to 600.degree. C. Both patents claim to produce a granular STP product that has low frangibility, high absorptivity and a bulk density in the range of about 0.65 to 0.87 g/cc. Both of these processes have the disadvantage of having an energy intensive calcination step.
U.S. Pat. No. 4,734,214 discloses a method of preparing powdered STP hexahydrate by spraying phase 1 STP and phase 2 STP separately with water, combining the two phases, suspending the mixture in water, and, finally, atomizing the suspension to produce the powder. The STP powder produced is said to have high absorptivity, low frangibility and good flow properties. This complicated process requires five steps to produce the desired STP product. Neither process yields a high absorptivity STP.
U.S. Pat. No. 4,800,055 discloses a process for making a granular STP wherein STP fines are hydrated and compacted to produce granular STP. Preferably, phase 1 STP fines are used. U.S. Pat. No. 4,394,358 discloses a process to make granular alkali metal diphosphates or triphosphates by hydrating the fines of the above phosphates, then granulating them in the presence of water and an ammonium polyphosphate.
U.S. Pat. No. 4,134,963 discloses a process to make low density granular STP by spraying STP fines with water in excess of that required for hydration to form an agglomerated product which is then calcined. U.S. Pat. No. 3,932,590 discloses a process for preparing medium density granular STP by spraying STP fines with a solution of sodium orthophosphate to prepare an agglomerated feed which is then calcined. These two processes also have the disadvantage of requiring calcination. None of the above patents disclose the present method of producing SP particles with high absorptivity.